Scholle, Peter A.¹, Troels Albrechtsen², Dana Ulmer-Scholle³ 
(1) New Mexico Bureau of Geology, NM Tech, Socorro, NM 
(2) Maersk Olie og Gas AS, Copenhagen K, Denmark 
(3) New Mexico Inst. of Mining and Technology, Socorro, NM

ABSTRACT: Geochemical Clues to Pressure History and Diagenesis of Chalks, Skjold Field, Danish North Sea

Preservation of substantial porosity in subsurface chalks is difficult because chalks compact rapidly during overburden loading. Overpressuring is known to be effective in retarding porosity loss and substantial ovepressures are widespread in North Sea chalks and overlying Tertiary shales. Studies indicate that overpressuring in the region surrounding the Skjold field existed for at least the past 10 million years, but local maintenance of overpressures for such periods of time is difficult in a field with diapiric salt movement. Partial or complete loss of overpressure at any stage after initiation should lead to rapid porosity loss and possible flushing of the chalks with deeper-basinal fluids. 
Geochemical studies, especially fluid-inclusion and stable isotopic investigations, of Cretaceous chalks of the Skjold Field in the Danish North Sea have provided insights on episodic variations in fluid compositions, and thus, by inference, into variations in fluid throughput and pore pressure history of that field. Successive generations of calcite cements in fractures from chalks on this salt dome structure show alternations between zones of water- and oil-filled fluid inclusions, sometimes with a corrosion zone between them. Those variations are interpreted to reflect at least two episodes of oil filling and overpressuring of the structure, followed by failure of the seal, with loss of overpressuring and flushing of the initial oil charge. Following those pressure dissipation events, calcite cementation under high-temperature aqueous conditions helped to heal the extensive fractures produced in the chalk. This allowed gradual repressurization and refilling of the structure with hydrocarbons.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.